Toners and process for preparing same

ABSTRACT

The compositions (toners) are useful in electrostatic printing and xerography and are a mixture of base and modifier particles with the base particles having a dielectric constant K and the modifier particles being of a normally solid, inert, nonhygroscopic, inorganic compound which compound has a dielectric constant K&#39;&#39; of at least 10 and sufficient to give a K&#39;&#39;/K ratio of at least 2 and an electropositive metal containing moiety chemically associated with an electronegative moiety. The metal of the electropositive moiety has a work function of up to about 2.5 electron volts. The toner has a charge/mass ratio at least about 50 percent greater than that of the base particles in their unmodified state.

United States Patent 1 Merck et al.

[ Dec. 18, 1973 TONERS AND PROCESS FOR PREPARING SAME [75] Inventors: John W. Merck, Tyrone, Pa.; John D. Nixon, Annapolis, Md.

[73] Assignee: SCM Corporation, New York, NY.

[22] Filed: Oct. 3, 1969 [2]] Appl. No.: 866,080

Related US. Application Data [63] Continuation of Ser. No. 633,642, Feb. 23, 1967, abandoned, which is a continuation of Ser. Nos. 447,758, April 13, 1965, abandoned, and Ser. No. 447,759, April I3, 1965, abandoned.

[52] US. Cl. 252/62.l [51] Int. Cl G03g 9/00 [58] Field of Search 252/62.]

[56] References Cited UNITED STATES PATENTS 2,529,566 ll/l950 Monack 252/632 2,797,705 5/1957 Vieweg 252/635 UX Wielicki 96/l Cooper 252/621 57 ABSTRACT The compositions (toners) are useful in electrostatic printing and xerography and are a mixture of base and modifier particles with the base particles having a dielectric constant K and the modifier particles being of a normally solid, inert, non-hygroscopic, inorganic compound which compound has a dielectric constant K of at least 10 and sufficient to give a K/K ratio of at least 2 and an electropositive metal containing moiety chemically associated with an electronegative moiety. The metal of the electropositive moiety has a work function of up to about 2.5 electron volts. The toner has a charge/mass ratio at least about 50 percent greater than that of the base particles in their unmodifled state.

8 Claims, 4 Drawing Figures PAIENIEUHEE 18 I915 I SHEET 10F 2 Fig. 2

TONERS AND PROCESS FOR PREPARING SAME This application is a continuation of US. Pat. application Ser. No. 633,642, filed Feb. 23, 1967, now abandoned, which is a continuation of US. Pat. applications Ser. No. 447,758 filed Apr. 13, 1965 and Ser. No. 447,759 filed Apr. 13, 1965 both now abandoned.

This invention relates to improved toners used in electrostatic printing. This invention relates to improved dry powders (toners) of the type which are used in forming images electrostatically, for example, by passing powder through the openings of a' mask or stencil and transferring them across an air gap to a substrate on which they define the desired image. US. Pat. No. 3,081,698 describes typical apparatus of this kind, and its disclosure is here incorporated by reference. For the sake of brevity, this type of printing will generally be called spaced mask printing" hereinafter. The invention relates also to improved dry powders adapted for use in analogous apparatus and processes wherein image-forming powder derived from a preformed loose-powder image carried on one substrate is transferred electrostatically across an air gap to a different substrate.

This invention also relates to a process for preparing said improved toners.

This invention relates to improved xerographic toners, wet or dry, used in xerography and related copying arts to develop latent or like electrostatic images and/or to provide visible images made by contact transfer processes.

It will be understood that xerographic (electroscopic) toners are currently used in the dry-copying art to develop (ite. render visible) latent electrostatic images formed in various ways on photoconductive surfaces. For this purpose the toners are very finely divided (e.g. have particle sizes from a fraction of a micron to 30 or 40 microns). Where dry toners are employed, they are charged (usually triboelectrically) and are cascaded or otherwise brought into intimate contactwith the substrate which carries a latent image. Any excess which does not adhere to the image (where this is desired) or to the area around the image (where this is desired) is recovered and used in developing a later latent image. Where wet" toners are used, they are suspended and dispersed in a voltatile, nonsolvating liquid and in such dispersed form are brought into contact with a substrate carrying a latent image. While the latent image which is to be so developed may be carried by a photoconductive substrate, it will be understood that-the dry or wet toners can also be applied to other substrates, e.g. transfer sheets, plain paper, etc., which are not photoconductive but have had a latent electrostatic image transferred to them in various ways, e. g. as by contacting an image-bearing photoconductive substrate with said transfer sheet, paper, etc. Such toners are usually dark and opaque, due to being compounded with finely divided solid colorant such as carbon black, or other dark pigments. However, by selection of the solid colorants, or of dyes, if desired, the toners can exhibit any desired color. For some special purposes they can be water-white and transparent.

After such toners whet-her modified in accordance with this invention or unmodified, have developed the desired electrostatic image on a desired substrate, or where they are transferred electrostatically across an air gap to a substrate, they are usually fixed" (permanently adhered) to the substrate. While fixing can be accomplished in various ways as by spraying an adhesive over the developed image, by spraying with a solvent for the toner, or by covering the developed image and substrate with an adherent sheet of transparent material (e.g. clear plastic) a more common way of fixing the developed image is by using toners which are easilyfusible (e.g. particles of rosin, shellac, thermoplastic synthetic resins(s), etc.) and heating the substrate and the toner which it carries until the toner fuses and becomes bonded to the substrate. Where heat resistant substrates permit the use of higher temperatures, then the toners can be made up of particles of glasses which soften and become adherent between temperatures such as 600F and 1,600F or higher.

For some purposes it is desirable to transfer toners in spaced mask printing and analogous situations so as to form a transitory (temporary) image on a desired substrate. After said transitory image has served its temporary purpose, the powders defining said image are removed in toto from said substrate. In such uses, there is no need to fix the image to the substrate. Hence, in accordance with the invention, the improved powders need not be fusible; i.e. they need not be thermoplastic organic materials such as synthetic resins, hard waxes, hard gums, rosin, etc., and need not be heat-softenable glass or frit. This is pointed out in detail to emphasize the fact that the present invention can properly be applied to any normally-solid particulate base material which can be transferred electrostatically across an airgap.

According to the invention, any organic and/or inorganic material in particulate form and otherwise appropriate for use as a wet or dry toner, hereinafter called the base powder," can be improved by modifying at least the surface of such powder with equally fine or finer particles of inorganic material( s) characterized by containing at least one electropositive metal having a low work function (below about 2.5 electron volts) and a dielectric constant of 10 or more sufficient to give a ratio of at least 2 between the dielectric constant K of the modifying particles to the dielectric constant K of the base particles.

In general, the preferred inorganic materials which exhibit the foregoing qualities are normally-solid metal salts which are non-hygroscopic and are inert chemically in respect to oxygen, water, the base material and the material constituting the substrate on which the improved toners are deposited to form an image. However, materials which in strict chemical parlance are not salts but which otherwise exhibit the foregoing requirements can be used equally Welland are part of the present invention. That is, the invention contemplates the use, as modifying material, of any material having a dielectric constant of 10 or more and an electropositive moiety containing at least one metal having a work function below about 2.5 electron volts.

The'chemical and physical attributes of such modifying powders will be explained in greater detail herebelow, but for the present purpose of presenting a generalized view of the invention, it should be understood that the invention contemplates either l) a simple admixture of the base powder(s) and the modifying particles, wherein the latter are distributed over the surfaces of the base powders, or (2) composite mixtures of the base toner composition (e.g. in molten condition) with the modifying particles so accomplished as to secure a mixed twoor poly-phase mass which can later be reduced to a powdery form and in such form (a) will have some modifying particles at or closely adjacent to the surface of the resulting powders and (b) have other modifying particles disposed interiorly in the resulting toner powders.

In respect to the dielectric constant of the material(s) used as a base for our improved toner, it will be recognized that the prior art has described a wide variety of materials as being suitable for use in making toner powders, most of the organic materials so described have relatively low dielectric constants (e.g. below about 2.5 Some of the glasses which have been described have higher dielectric constants. Any of such and other materials can be improved by practising the present invention since in accordance with the invention we select the modifying material so that its dielectric constant is at least and at least double that of the base material(s).

It will be understood that in so selecting a modifying material, the dielectric constant of the selected base material is considered to be the effective" dielectric constant. For example, where a base powder or base material has been compounded with e.g. pigment(s) and/or dyes to impart desired color, and/or with other solid particulate materials, the dielectric constant of such a compounded base powder or base material is not to be confused with the dielectric constant of any particular material which is present therein. In other words, it is the effective dielectric constant of the material as measured in gross.

Accordingly, it is an object of the invention to provide improved toners modified with particles (a) containing one or more metals having a work function below about 2.5 electron volts and (b) having a dielectric constant of 10 or more sufficient to give said ratio of dielectric constants of 2 or more. The modifying particles preferable are alkali metal-and/or alkaline earth metal salts of acids such as carbonic acid, sulfuric acid, titanic acid, silicic acid, zirconic acid, and others set forth hereinafter.

The invention will be understood more fully from the following description and illustrative embodiments of the invention taken in conjunction with the attached drawings, in which:

FIG. 1 is an isometric sectional view ofa greatly enlarged particle of the invention having a resinous base and having finer particles of modifier distributed at and on its surface;

FIG. 2 is a similar isometric view of a particle of the invention having a base of resinous material and having modifier particles within the resinous material as well as at and on the surface;

FIG. 3 is a similar isometric view of a particle of the invention having a base of glass and having modifier particles within the glass as well as at and on the surface; and

FIG. 4 is a schematic illustration of equipment used by us to measure the charge/mass ratio of toners in general and of our improved toners in particular.

It will be understood that electroscopic toners in the past have exhibited shortcomings in their ability adequately to develop fine detail of images, due in part to what can be described as poor free-flowing characteristics in the toner and in part to an unfavorable charge/- mass ratio of the toner. The toners of the present invention have been improved in both of these respects. In addition, the toners of the present invention exhibit other merits including superior performance because of their ability to respond to lower potentials and potential gradients. All of the improved qualities of the subject toners result in improved visual quality in developed xerographic images. Moreover, the improved toners, when prepared from base toner compositions which are fixed to an image substrate by fusion, retain their fusibility in substantially undiminished quality.

Referring now to FIGS. 1-3 of the drawings, it will there be seen that typical powders particles which constitute our improved toners are normally-solid base particles 1, 3 or 4 which have been composited with modifying particles 2, 2. In FIGS. 1 and 2, the base particles are shown to be composed of resinous material. In FIG. 3 the base particle is shown to be composed of glass. However, the identification of the base particles as being resinous or glass should not be construed as limiting the invention, since any normally-solid organic and/or inorganic material in particulate form can be used as a base material.

The Base Powder(s) or Composition(s) It should be understood that the invention is not limited to the use of any particular classes or kinds of base toners or compositions. Thus, the principles of the invention can be applied to any of the thermoplastic resinous powders and compositions which have been described in the prior art as being appropriate for developing xerographic and Iikeimages and which can then be fixed by fusion, e.g. at temperatures up to about 300F. In this respect it should be noted that such powders and/or compositions can be permanently thermoplastic (repetitively fusible without deterioration) or they can be of such chemical nature that they pass through a thermoplastic stage before becoming theremoset. Moreover, the base powder or composition can consist solely of such thermoplastic resinous materials, or such materials can be or have been compounded with finely-divided solids such as fillers, pigments, dyes, etc. whereby they can be or provide clear essentially colorless toners or toners which have any desired color and any desired degree of translucency up to and including opaqueness. Typical resinous materials are rosin, shellac, hard waxes, hard gums and synthetic resins of diverse chemical natures.

It should also be understood that the base powder(s) or composition(s) can be non-resinous, i.e. inorganic, while yet being fusible for fixing, e.g. glass or glass-like inorganic compounds, solid solutions and/or reaction products fusible at e.g. about 600F-l ,800F or higher. These powders or compositions, like the thermoplastic resinous powders and compositions, can be essentially single-phase in physical appearance, or can be composites of two or more phases wherein finely-divided solids impart color, translucency, opaqueness, and/or (as will be apparent below) modification of dielectric constant.

For special purposes, toners which are essentially colorless and clear are sometimes used to provide a temporary or transitory development of an image. After the temporary image has served its purpose, the toner is removed in toto from its carrying substrate. For such purposes the toners need not be fusible nor otherwise fixable to the substrate.

The present invention applies to all of the foregoing classifications of toners, and applies whether the toners are dry powders or are in the form of pastes or liquids, i.e. wet toners.

It will be understood that toners, as described above, are finely-divided so as to be able to differentiate boundaries between strongly-charged, weakly-charged and uncharged areas of an image-carrying substrate. Currently, the particle sizes vary widely between a fraction of a micron and 30-40 microns or more, depending somewhat on convenience and somewhat on the desired faithfulness of development.

Where wet toners are sought, the dry toners can be dispersed in a wide variety of volatile, inert and usually non-solvating solvents (with respect to the toners), with or without dissolved and/or solvated adjuncts such as film-formers, thixotropic agents, adhesives, etc.

It willalso be understood that the described toners of the past (whether dry or wet) have usually been of such a a nature that the dielectric constant has been below 8 m9, more usually below about 5 (eg 2-3). In this respect, it should be noted that the dielectric constant of toners which are composites of two or more separate materials is, for the purposes of the present invention, the effective" dielectric constant of the composite, as measured in gross. That is, the dielectric constant of such a composite should not be confused with the dielectric constant of a particular material contained in the composite.

The Modifying Particles As pointed out above, the modifying particles must exhibit two significant qualities, namely, low work function and a dielectric constant of 10 or more.

As used herein, work function is the sciencerecognized attribute associated with materials, and in the present invention it refers to the metal(s) contained in the electropositive moiety of a material being considered for use as a modifying material. Metals 'of Groups Ia and Ila, as identified on pages 448 and 449 of the Handbook of Chemistry and Physics, 40th Edition, Published by Chemical Rubber Publishing Co., Cleveland, Ohio, Copyright 195 8, (i.e. Li, Na, K, Rb, Cs, Ca, Sr, Ba and Ra) are known to exhibit work functions below about 2.5 electron volts, as measured by the contact potential method (See Advances in Catalysis" Vol. VII, Pgs. 3l3315). These are the metals which must be present in the electropositive moiety of the modifying materials used in the practice of this invention.

The electronegative moiety of the modifying materials is typically but not restrictively a residue of one or more of certain inorganic acids., i.e., an-inorganic acid residue consisting of carbon and a chalcogen, of two different chalcogens, or of a chalcogen and a metal of the group consisting of silicon and the metals of Groups IV!) and Vb, as identified on pages 448 and 459 of the Handbook of Chemistry and Physics, 40th Edition, Published by Chemical Rubber Publishing Co., Cleveland, Ohio, Copyright 1958, (titanium, zirconium, cerium, hafnium, vanadium, columbium and tantalum).

However, in addition to the foregoing attributes of the electropositive and electronegative moieties of the modifying material, the said salts must be further restricted, for functional reasons, to salts which are nonhygroscopic (i.e. unable to absorb or condense moisture from the ambient atmosphere) and are chemically inert to oxygen, water, the base material and/or the material of the substrate to which the improved toners of the invention are applied. Said salts must, of course, be normally solid, and preferably should be non-toxic.

The following list illustrates (without limiting) the electronegative moieties contemplated for use with alkali metal-containing and/or alkaline earth metalcontaining electropositive moieties. The electropositive and electronegative moieties need not be in stoichiometric proportions. M is used to identify any one or more of the metals of Groups In and Ila of the Periodic Table, without designation of its valency, and x and y represent small integers (e.g. l, 2, 3 etc.) or fractions of such integers.

Such and other similafiactioh prodfitsifiicompassed by the definitions set forth under the sub-heading The Modifying Particles, can also be doped with small mounts of different compounds known to modify their dielectric constants. See Ceramic Age, Dec. 1, Pgs. 16-19, here incorporated by reference.

Preparing The Improved Toners It will be understood that the improved toners are composites of the modifying material(s) and of the base material(s). For most purposes, the modifying material(s) are used in amounts below about 50 percent by weight, based on the weight of the base material(s). However, as will be pointed out below, the weight proportions used can be higher and usually are much lower since said proportions are much less significant as a parameter of the invention than is the distribution of the modifying material in and/or on the finished composite toner particles. In other words, the improvement of base particles by the modifying material(s) is best identified in terms of the charge/mass ratio of the finished composite toner particles established during the printing process. That is, the charge/- mass ratio of the base particles (free of modifying material of the invention) can be measured electrically and then compared with the charge/mass ratio of modified (composite) toner particles using the same base material. In accordance with the present invention, the charge/mass ratio of the modified toner particles should be at least 50 percent greater than that of the unmodified base particles, and preferably should be at least double. Five-fold, ten-fold and higher improvements in charge/mass ratio can readily be accomplished, and in numerous instances such improvements bear no direct linear relation to the weight proportions between modifying material(s) and base material(s).

The foregoing disparity between charge/mass ratio and weight proportions will be better understood when it is realized that a surface effect appears to be involved in the measured charge/mass ratio of the modified particles since modifying particles at, on or near the surface of the toner powders appear to be somewhat more effective than are modifying particles disposed interiorly in said toner powders.

The modifying particles preferably have an average particle size below about I micron, and should preferably be finer than are the base toner powders (typically 1-40 microns) particularly when the modified (composite) toner powders are prepared by simply mixing the two kinds of powder together. However, modifying particles and base toner powders of about equal sizes (eg about 1 micron) can also be so mixed, and the resulting composites can exhibit at least a 50 percent increase in charge/mass ratio when K/K ratios of 2 or more are involved by the selected materials.

When modified fusible toner powders corresponding to the invention are prepared by a. melting a selected base material (which can be a single fusible-resin, a glass, a mixture of resins, and- /or a like fusible mass which has previously been mixed with carbon black or other solid colorants or y b. mixing the modifying particles into the molten mass, and

c. comminuting the resulting composite mass by any desired method, with or without an intermediate solidifying step, then, of course, the modifying particles so added to the molten base material should preferably be no larger than the size of the modified powders which are to be produced. Otherwise the process of production will entail an operation wherein the starting modifying material must be further reduced in size after it has been mixed with the base material.

When modified glass-base toners are to be prepared, a procedure such as set forth in the preceding paragraph can be used. However, in some cases it is also possible to formulate a glass so that after it has been smelted and fritted, it will be a particulate form of a solid solution from which the desired modifying particles can be precipitated at or near the surface (and interiorly if desired) by a suitable nucleation or equivalent heat-treatment. That is, by the heat-treatment, the original single-phase particles are converted to a twophase or poly-phase state at least at or near the surface.

Other methods which can be used to prepare modified toners corresponding to the invention are:

l. fluo-solid type mixing of modifying particulate material(s) with selected base powder(s),

2. condensing aerosol-type dispersions or vapors of the modifying material onto the surfaces of the selected base powder( s) while the latter are maintained in a turbulent or agitated state,

3. atomization of the molten base material or composition into a cloud of the modifying particles, where the atomized material solidifies.

Where the modifying particles and base powder are simply mixed together, any suitable equipment such as blade mixers, tumbling barrels, V-can mixers, etc. can be used.

The effectiveness of the amount of modifying material and/or of the method employed to prepare the modified toners can best be determined by measuring the charge/mass ratio of said modified powders. Flt]. 4 illustrates, schematically, the equipment which we have used in making such measurements. Numeral l identifies a laboratory-type electrostatic spaced-mask printing unit (see US. Pat. No. 3,08l,698) consisting of a metallic base electrode 11, a removable parallel-faced insulating spacer 12 (Teflon or the like) and a removable top electrode which is made up of a screen 13 (typically 4 X 6 inches) mechanically and electrically secured to a heavy metal rim 14. The screen is preferably tightly-stretched in all directions while being secured, as by soldering, to rim 14, and is a 200 mesh stainless steel screen. If desired, portions of the screen 13 can be blocked out to define a desired pattern of opening through which applied toner can pass when a test is being made. The whole assembly, 10 can be rested on any suitable working support which insulates electrode 11 from ground.

Rim 14 of the top electrode is connected to one terminal of any suitable high voltage supply which can be turned on and off conveniently. The other terminal of said supply is grounded.

Base electrode 11 is connected to a terminal 15 of an electrometer 16 of conventional circuitry having a single pole double throw switch 17 therein which in one position grounds terminal 15 and in the other position connects a resistor 18 between said terminal and ground. Exteriorly of such a conventional electrometer a capacitor 19 is connected between said terminal 15 and ground. The other terminal 20 of the electrometer is connected to the input terminal 21 of a conventional oscilloscope 22. As will be understood, signal applied to terminal 21 provides two phased inputs A and B which when amplified by the preamplifiers and applied as A minus B to a cathode ray tube 23 controls the vertical motion of the electron beam of said cathode ray tube.

Numeral 24 identifies a typical trace appearing on the viewing screen of tube 23 when a test for mass/- charge ratio is being made. Such a trace can be photographed, if desired, to give a permanent record.

The capacitor 19 is included in the circuitry to collect a charge therein which is equal to the total amount of charge transported across the gap of the printing unit 10 by toning powder when such power is wiped through the screen. Such wiping can be done with a velvet covered Teflon blade having a uniform load applied thereto during the wiping motion. Under the described conditions, the voltage developed across capacitor 19 is proportional to the charge collected therein, and is conveniently displayed on the screen of the oscilloscope so that the electrical effects of the entire printing operation can be observed visually. Resistor l9 typically has a value of about l0 ohms.

The actual sequence of steps involved in making a determination are: (1) a preweighed substrate, desirably thin aluminum foil, 25, is placed on the bottom electrode 11 and held in place by spacer 12; then the screen electrode l3, 14 is positioned on the Teflon spacer (a preferred gap thickness is 0.090 inch) after which a small mound of toning powder 26 is placed on the screen 13. (2) high D.C. voltage is applied to the screen electrode 13, 13 (desirably -3.5 Kv for 0.090 inch spacing) with switch 17 in its grounding position, then switch 17 is changed to its other position and the resultant voltage level across the capacitor 19 is noted; (3) powder 26 is wiped uniformly across the screen using a single pass of the wiper blade; the new voltage level across the capacitor is noted; (4) the aluminum foil substrate upon which the powder has been deposited is then reweighed to obtain the mass of powder (M) transported across the gap.

Since the voltage (V) developed across a capacitor 19 is related to its charge Q by the relation,

where C is a constant equal to the value of capacitance of capacitor 19,

the difference between original and final voltage levels (AV of 24) need only be multiplied by C of the capacitor, and divided by the mass of powder (M) transported in the test to obtain the charge to mass value (Q/M C AV/M) which is characteristic of the powder for the measurement conditions prevailing. It is apparent that the higher the charge to mass ratio, the more effective is the powder for response to electrostatic fields and/or charges, i.e., electrostatic printing functions.

It is obvious that in order to compare the relative performance of electroscopic powders, whether modified or not, the previous steps need only be performed identically for each powder and the resulting Q/M values compared. For example, the following Q/M values have been obtained using this technique for a series of powder formulations composed of a commercial electroscopic toner as base, modified with various concentrations of BaTiO; powder (I( of about 1,000; average particle size below about 1 micron) by simply mixing the two powders together in a mechanical mixer. The tests used a voltage of -3.5 Kv, a gap of 0.090 inch and a 200 mesh screen.

Formulation Relative Q/M" A. Commercial toner alone" 1.0 B. A+ 0.5% by wt. BaTjO 6.6 C. A l.5% by wt. BaTlQs l4,7 D. A by wt. liaIiQ 18.8

*ratio of Q/M for BaTiO -modified toner Q/M for unmodified commercial toner a copolymer of styreme, a vinylic ester and polyvinyl butyral mixed with about 2% of carbon black and dispersed in the particles.

The following examples illustrate the principles of the invention and include the best modes presently known to use for practicing those principles in accordance with the invention.

EXAMPLE 1 EXAMPLE 2 Polyethylene toners (Microthene), are modified by mixing with said barium titanate of Example 1.

One of the polyethylene toners has a density of 0.915, a melt index of 22 and an average particle size 60 below microns.

Another of said toners has a density of 0.924, a melt index of 5 and an average particle size below 30 microns.

A third of said toners is like the first except it has been composited with about 2 percent carbon black to make it opaque and black.

When samples of each of the three toners are modified with 0.2 percent, 0.8 percent and 1.5 percent of the barium titanate by simply mixing the powders together, at least a two-fold increase in charge/mass ratio is measured.

When the modified and unmodified toners are used in preparing prints in xerographic or spaced-mask printing equipment, two benefits of the modification are easily noted; namely, (1) the flow characteristic of the modified powders are significantly better than those of the unmodified toners, and (2) the printed images prepared with the modified powders are noticeably sharper in detail and cleaner (i.e freer of particles which have strayed from the true boundaries of the image or from the projected boundaries defined by the mask).

In similar tests wherein the amount of barium titanate is raised progressively to about 10 percent by weight, based on the base toner powders, there is some progressive improvement in toning behavior with increasing amounts of barium titanate, as qualitatively judged visually, but it appears that optimum improvement, costwise, accompanies the use of less than about 5 percent (typically about 2 percent) of barium titanate.

EXAMPLE 3 When the tests of Example 1 are repeated except to use lithium columbate, potassium zirconate, barium sulfate, barium carbonate, barium zirconate, strontium titanate (BaQ, SrQ 0 (TiO2 )1.4 cobalt titanate, cobalt zirconate or bismuth titanate in place of the barium titanate, the resulting visual images clearly show that the titanates having electropositive moieties of low work function (i.e. lithium, potassium, barium, calcium, and strontium) give better prints than do the titanates or zirconates having electropositive moieties containing metals of high work function (i.e. bismuth and cobalt).

EXAMPLE 4 For test purposes, a commercial xerographic copying machine is run until its charge of toning powder is nearly exhausted. Said toning powder is carried in a closed chamber wherein a rotary brush and coarse resin particles are kept agitated, whereby a cloud of triboelectrically-charged toner is maintained adjacent a moving web carrying latent electrostatic images. When the chamber contains so little toner that images are being developed poorly, if at all, there is introduced some improved toner duplicating the fourth formulation D above, (5% BaTiO plus commercial toner). Shortly thereafter, a visual comparison of copies made before and after the introduction clearly showed that addition of the barium titanate to the toner improved the resolution of fine details in the copies.

EXAMPLE 5 A modified glass-base toner is prepared from commercial glass toner powder made of an stray of toner particles.

EXAMPLE 6 Wet toners are prepared by mixing commercial polyethylene toner powder with 2-5 wt. of the barium titanate of Example 1 and then dry grinding the mixture to impact the barium titanate into the surface portions of the polyethylene particles. The resulting composite particles are then dispersed in hexane to form a dispersion containing about 10 wt. of composite particles. The resulting dispersions are tested in a commercial reproduction machine wherein the web carrying latent electrostatic images is dipped below the surface of an agitated developing bath and then passes to a drying station where the hexane is evaporated. No attempt is made to recover the hexane, but this can be done if so desired.

The tests show that the modified toners give sharper and cleaner toned images than the comparable wet toner wherein the polyethylene particles are free of added barium titanate.

In summary, it will be understood that the improved toners of the invention, whether prepared from base powders or base compositions which are organic in nature of inorganic, and which are essentially clear and transparent at one extreme, or translucent or opaque and of any color at the other extreme can be improved with our modifying particles by simply mixing the base powders and modifying particles together, or by incorporating the modifying particles within the particles of base powder. While simple mixing gives modified powders which appear to function as though the modifying particles are firmly adhered to the base particles, we want to make it clear that we do not presently understand the precise nature of said apparent adherence. it may be that as long as the base powders and the modifying particles are transferred electrostatically at equal rates across an air-gap, the over all effect is the same as if each modified particle performed like an integrated unit in said transfer.

While we have expressed preferences through the foregoing description of the invention, it is helpful here to state our special preferences; namely, fusible polyethylene powders (or similar thermoplastic polymers of lower olefins) as base powders, and calcium titanate, barium sulfate and/or barium carbonate as the modifier(s). While barium titanate is an especially. good modifier, it is potentially toxic and hence less desirable than calcium titanate. Costwise, both barium sulfate and barium carbonate offer an advantage over the titanates.

What is claimed is:

1. An improved toner consisting essentially of, a mixture of base particles and modifier particles, said base particles having a dielectric constant K, said base particles being materials selected from the group consisting of rosin, shellac, hard waxes, hard gums and synthetic resins, and said modifier particles being particles of at least one normally-solid, inert, non-hygroscopic, inorganic compound having (A) a dielectric constant K of at least 10 and sufficient to give a KIK ratio of at least 2, and (B) an electropositive metal-containing moiety chemically associated with an electronegative moiety, said modifier particles being present in an amount below about 50 percent by weight based on the weight of base particles, said electropositive metal-containing moiety being selected from the group consisting of the metals of Group la and Group [1a of the Periodic Table, said electronegative moiety being selected from the group consisting of titanates, but excluding barium titanate, zirconates, silicates, ceriates, thoriates, hafniates, vanadates, columbates, tantalates, carbonates, sulfates, sulfites, selenates, selenites, tellurates, tellurites, thiotitanates and thiozirconates, the metal of said electropositive moiety having a work function of up to about 2.5 electron volts, said improved toner having a charge/mass ratio at least about 50 percent greater than that of said base particles in their unmodified state, said base particles having a size of between 1 and 40 microns.

2. An improved toner as claimed in claim 1, wherein said mixture comprises composite particles wherein the modifying particles are located substantially only on the surface or are located both on the surface and interiorly of the composite particles.

3. An improved toner as claimed in claim 1, wherein said mixture comprises base particles with modifier particles essentially on the surface thereof.

4. An improved toner as claimed in claim 1 wherein said mixed particles are disbursed in and carried by a liquid phase which is volatile and inert, and nonsolvating in respect to said mixed particles.

5. An improved toner as claimed in claim 1, wherein said modifier particles have an average particle size below about 1 micron and are finer than said base particles.

6. An improved toner as claimed in claim 1, wherein said charge/mass ratio of the toner is at least double that of said unmodified base particles.

7. An improved toner as claimed in claim 6, wherein said base particles are fusible.

8. An improved toner as claimed in claim 6, wherein said base particles are pigmented so as to be dark in color.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,779,92 Dated December 18, 1973 Inventor-( s) 1 1m u I Merck; Qohn Q. Nixon It: is certified that eiror appears in the aboveidentified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 45, for "volqatile" should read --volatile;

Columh 4, line 36, for "theremoset" should read -thermoset--;

Column 12, line'37, for "disbursed" should read -dispersed--.

#"s n d and sealed, this 16th day of July 197A.

(SEAL) Attest:

MCCOY M. GIBSON, JR. c. MARSHALL DANN Attesting Officer Commiss oner of Patents 

2. An improved toner as claimed in claim 1, wherein said mixture comprises composite particles wherein the modifying particles are located substantially only on the surface or are located both on the surface and interiorly of the composite particles.
 3. An improved toner as claimed in claim 1, wherein said mixture comprises base particles with modifier particles essentially on the surface thereof.
 4. An improved toner as claimed in claim 1 wherein said mixed particles are disbursed in and carried by a liquid phase which is volatile and inert, and non-solvating in respect to said mixed particles.
 5. An improved toner as claimed in claim 1, wherein said modifier particles have an average particle size below about 1 micron and are finer than said base particles.
 6. An improved toner as claimed in claim 1, wherein said charge/mass ratio of the toner is at least double that of said unmodified base particles.
 7. An improved toner as claimed in claim 6, wherein said base particles are fusible.
 8. An improved toner as claimed in claim 6, wherein said base particles are pigmented so as to be dark in color. 